Most new regiments come from a single planet, whereas the older ones will usually be made up of the remnants of lots of nearly destroyed regiments that are all rolled into 1. Conscripts are ALWAYS from the local area, because the Imperium will just go around the area and say "right, all men that can hold a gun, congratulations, you're now in the Military. Take this armour, this gun and try to kill those things over there. You'll know the enemy because most of them have more teeth than the entire Osmand family.
More simply put, the 'vanilla' Guardsmen regiment is from Cadia. Obviously they only provide a tiny fraction of Imperial Guardsmen, but they're pretty staple and have the most Guardsmeny Guardsmen there ever was. That's important to note.
Every Guardsmen Regiment is unique in who is in it, how it operates, its history, and its equipment. The Imperial Tithe draws manpower from across the stars, creating Regiments to bring the Emperors word to heathens, rebels, xenos, and worse. In that way, while there's standard equipment and the like, most Regiments do not work like that.
A regiment may be composed completely of nobility, thus be equipped with items and artifacts that'd make anything but an Astartes or the Inquisition blush. They may ride great Xeno's beasts and be drawn from a world full of cave men; they may be simple Farmers; They may be an army of thieves or mercenaries. The list goes on. However, Regiments are always specialized at just one thing. For example, the Fifth (Insert planet they came from here) Mechanized Infantry Regiment would, well, be mechanized infantry. No regiment has the capacity to fully use combined arms en masse on their own. They need other regiments to work with them so they can do that; this is primarily so it'll be hard for regiments to defect and be a capable fighting force against the Imperium. This is because of the Horus Heresy.
That is why those commanding Guard Regiments must work together; that's just how the Guard works. Alone, you are nothing. Together and with proper leadership, the Guard become "The Hammer of the Emperor" in every sense of the word.
The Astartes operate differently; a meagre force of 100 can cut through 20k Guardsmen like butter, but they have serious issues regarding over extension, since they're so tiny. Defensive Turrets simply will not cut it here. They aren't fighting Orks who will be stalled by sentry guns while the Marines go on the offensive. Further, this is a well armed and organized military force, capable of matching your average Imperial Guard regiment or exceeding it. They can pull a lot of nasty tricks on the Marines, like bombarding their position and just not actually fighting them. Losing a couple thousand men for one Astartes is a fair trade.
The Marines cannot do this alone, either. A well organized, well equipped rebellion in the Imperium is very fearsome. It is essentially inevitable that they'll lose. At the end of the day, the Imperium can send trillions of troops if need be. The question is, will we be the ones to make them lose? Because unless we do a good job of this, it could turn into a decade upon decade siege of the entire Consortium, and most of us will die in that time span.
1x Thank
